In U.S. Pat. No. 4,883,705, some lead-free mineral glass compositions are also described within the context of the manufacturing of ceramic substrates used for electronic devices. Said compositions are characterized by a relatively low SiO2 content (38 to 48% by weight), a relatively high ZnO content (10 to 20% by weight), a relatively high CaO content (1 to 8% by weight) and a relatively low Li2O+Na2O+K2O content (0 to 5% by weight).
Most of the commercial glass-ceramic cooktops are decorated with the aid of variously coloured enamels. Such a decoration on the one hand conforms to aesthetic criteria, which imply a large variety of tints and motifs, and on the other hand, aim to attain the objective of safety, in outlining with these decorations the location of the areas which heat and which, during the functioning of the cook top, are susceptible to attaining high temperatures.
The preparation of the enamels is normally done in several steps. First of all, a glass is melted according to classical glass industry techniques. This glass is then ground to a particle size which is compatible with the subsequent technique of application of the enamel onto the support to be enamelled (an application technique which generally consists in screen printing or decal(comania)). Pigments and/or opaque-making agents are added to the glass powder which are intended to confer to the final enamel its tint and its opacity. The resulting powdered mixture (glass powder+pigments and/or opaque-making agents) is finally incorporated into an organic binder, in proportions which are adjusted to the viscosity desired for the resulting paste. The enamel composition thus obtained is ready for use.
The enamel composition is applied onto the substrate concerned, by direct screen printing, by transfer, by decal(comania) or by any other process, and then dried and fired by an appropriate heat cycle, which typically comprises a plateau at a temperature of greater than 900° C.
Beyond the two functions set forth above, (with reference to the aesthetics and to the safety), the enamel decorations must meet the physico-chemical requirements which arise from the conditions of use of the glass-ceramic cooktops, cooktops on which said decorations generally are incorporated. Thus, the decorations must be able to resist attack by acids and bases; they must be easy to clean, even after calcination of food remains; they must have a good resistance to abrasion, to friction with metallic pieces; they must not be sensitive to wrenching. The incorporation of said decorations must not either affect, too much, the mechanical strength of the tops on which they are incorporated. To the present day, certain tops on the market are susceptible to being broken, when a pressure of the order of 70 to 80 MPa is exerted on the enamelled face. Under certain conditions, especially of extreme transport and of storage, a resistance to a greater pressure (120-140 MPa) is required from the tops.
Traditionally, the introduction of lead oxide into the glass composition (entering in the composition of the enamel) strongly lowers its viscosity and thus guarantees a good coating, i.e. a good moistening and a good covering of the glassceramic by the enamel. However, in view of the present tendency to remove toxic elements from products used in cooking, we consider that the enamels used for the decoration of glass-ceramic cooktops must be free from such elements, and in particular free from heavy metals, such as lead.
According to prior art, within the context of a search for very efficient products, numerous lead-free glass compositions and enamel compositions which incorporate said glasses have been described.
In FR-A-2 686 333, glass compositions are described which are lead-free and cadmium-free and which are intended for vitrification, enamelling and decoration. Said compositions contain no lead, no cadmium, no zinc, no tin, no fluorine. They can in particular contain barium, between 0 and 12% by weight of barium oxide. Finally, they are characterised by a relatively high CaO content (3 to 18% by weight).
The Applicant has itself, in FR-A-2 732 960, described lead-free enamel compositions which comprise finely divided glass particles, finely divided pigment particles and an organic binder, which are characterised in that the glass has a thermal dilation coefficient less than or equal to 55 10−7/° K and has the following composition, in percentages by weight, on the basis of the oxides:
SiO245-60B2O3 0-10Al2O3 6-17Li2O0-2Na2O0-3K2O0-3Li2O + Na2O + K2O<4CaO 0-12MgO0-9ZnO 0-17BaO 0-27SrO 0-16CaO + MgO + ZnO + BaO + SrO22-42TiO20-2ZrO20-7said glass having a softening temperature TL greater than 680° C.
The Applicant has pursued its research work and is now in a position to be able to provide novel lead-free enamel compositions containing particles of lead-free mineral glass and the corresponding novel enamels.